Successful overwintering of small mammals in temperate and cold climates requires behavioural and
physiological adaptations. There are several strategies to survive food shortages and the cold.Most species
of small mammals use multiple methods simultaneously but nest building and burrowing are the most
widespread among them. A well-constructed, dry nest insulates animals from harsh ambient conditions.
Mound-building mice build large banks in the autumn and establish a burrow system with nest chambers
beneath them. These overwintering structures are built from soil and a considerable amount of plant
material. Recent studies presume that the stored vegetable matter does not, or not exclusively, serve as
food and indicate that the mounds might have insulating role. To investigate the function of their plant
fill, we built artificial mounds with varying plant content, similar to those built by mound-building mice.
We measured temperature change at three levels, at the surface, under the mound and at the nest depth,
and investigated their water retaining properties. We showed that the plant fill plays a major role in their
thermal insulation and waterproofing properties. Mounds reduced temperature variation of the soil and
may protect the nest from absorbing precipitation during the winter.